Fusion

Discussion

introduction

The sun has been around for some five billion years and is expected to shine for another five billion years to come.

gravitational energy would last about 30 million years

chemical energy would last about … well, this is a stupid comparison. What reactions are possible with pure hydrogen? H + H = H2? Helium is inert. Plus, it's too damn hot in the sun for chemical reactions.

Size is related to energy. Nuclear energy is to chemical energy as atomic dimensions (10−10 m) are to nuclear dimensions (10−15 m). Nuclear reactions have energies on the order of 100,000 times the energy of chemical reactions.

Paraphrase needed …F. W. Aston discovered in 1920 the key experimental element in the puzzle. He made precise measurements of the masses of many different atoms, among them hydrogen and helium. Aston found that four hydrogen nuclei were heavier than a helium nucleus. This was not the principal goal of the experiments he performed, which were motivated in large part by looking for isotopes of neon. The importance of Aston's measurements was immediately recognized by Sir Arthur Eddington, the brilliant English astrophysicist. Eddington argued in his 1920 presidential address to the British Association for the Advancement of Science that Aston's measurement of the mass difference between hydrogen and helium meant that the sun could shine by converting hydrogen atoms to helium. This burning of hydrogen into helium would (according to Einstein's relation between mass and energy) release about 0.7% of the mass equivalent of the energy. In principle, this could allow the sun to shine for about a 100 billion years. In a frighteningly prescient insight, Eddington went on to remark about the connection between stellar energy generation and the future of humanity:

If, indeed, the subatomic energy in the stars is being freely used to maintain their great furnaces, it seems to bring a little nearer to fulfillment our dream of controlling this latent power for the well-being of the human race — or for its suicide.

Bethe described the results of his calculations in a paper entitled "Energy Production in Stars".

Light nuclei join to form a heavier nucleus. Energy is released in the process. Fusion powers the stars and high yield thermonuclear weapons.

solar fusion

Stars begin as a cloud of mostly hydrogen with about 25% helium and heavier elements in smaller quantities. The sun, 107 K core, hydrogen fuses to form helium through a process known as the proton-proton chain (often shortened to the p-p chain).